There is an increasing interest in the development of a compact diode laser source in the 400-500 nm range for advanced applications, such as the enhancement of optical disk technology through improved data storage, retrieval storage density, and data capture rates.
There are several possible approaches to a prospective short wavelength, diode laser source. The traditional diode materials such as the ternary and quaternary compounds of In, Ga, As, Al, P, and Sb do not have a direct energy gap high enough to produce a short wavelength laser. A number of other laser materials have been studied for development of diode laser sources in the blue wavelength region. These materials include II-VI semiconductors, quantum well materials, and other wide band bap semiconductors such as cadmium sulfide (CdS). Although the blue quantum well structure shortens the lasing wavelength, it requires a cryogenic condition for lasing. There also has been investigation of II-VI wide-gap superlattices with the goal of achieving diode emission in the blue-green region of the spectrum, but these materials have only produced optically pumped lasers requiring cooling at liquid nitrogen temperature. Other semiconductors that are known to lase in the 450-500 nm range are CdS, ZnSe, ZnCdS, and CdSeS. Lasers of these materials require either optical or electron beam pumping for their operation.
In view of the impracticality of a direct diode laser source, attractive alternatives involve frequency conversion of available diode laser sources, either by frequency doubling or parametric up-conversion. The traditional frequency conversion techniques utilize phase-matching of input beams and harmonic waves in second order optical crystals such as potassium dihydrogen phosphate (KDP and KD*P), LiNbO.sub.3 and LiIO.sub.3. However, because of the relatively low values of second order susceptibility of these crystals, and the low beam intensity of a diode laser, an exceptionally long single crystal is required to achieve appreciable power conversion to the second harmonic tensor. Such large crystal dimensions preclude the design and fabrication of a compact and ruggedized optical recording system. In addition, the provision of large inorganic crystals is difficult and costly.
In general, classical phase-matching (e.g., via angle or thermal tuning) requires a certain combination of intrinsic birefringence and dispersion of refractive indices. New small molecular weight crystralline organic nonlinear optical materials with high second harmonic susceptibility have been reported in literature such as ACS Symposium, Series No. 233, pages 1-26, 1983 by Garito et al. These organic materials usually possess high intrinsic birefringence and positive dispersion so that phase matching can be achieved with a single crystal. Even if phase matching can be achieved with the new types of organic materials having high nonlinear optical susceptibility, the low beam power of a diode laser significantly limits the power conversion efficiency.
An alternative means to achieve phase-matched conditions is the use of dispersion properties for different modes in a waveguide. Since the energy is confined laterally to a narrowly constricted space, a high light intensity is possible with a relatively low power source. In this approach, one usually excites a lower order mode of the fundamental beam and the second harmonic generated propagates in a higher order mode. If the waveguide geometry and refractive indices of the guiding region and cladding region are such that: EQU .DELTA..beta.=.beta..sub.n (.omega..sub.3)-.beta..sub.m (.omega..sub.2)-.beta..sub.1 (.omega..sub.1)=0 (1)
then the phase matching condition is established. Here, .beta..sub.i is the propagation constant of the i-th mode. The conversion efficiency is generally quadratically dependent on the overlap integral between the two modes; EQU .intg.E.sub.n (.omega..sub.3, z)E.sub.m (.omega..sub.2, z)E.sub.1 (.omega..sub.1, z)dz
where E is the electric field of the mode across the waveguide. In general, the overlap between the waveguide modes is limited, and the value of the overlap integral is also small. This approach has been utilized for phase matching in waveguides derived from organic materials, as reported in Optics Commun., 47, 347 (1983) by Hewig et al. However, the level of second harmonic conversion efficiency is too low for any practical frequency doubling application.
Of particular interest with respect to the present invention is literature relating to spatially periodic nonlinear structures for frequency conversion of electromagnetic energy. The pertinent literature includes IEEE J. of Quantum Elect., QE-9 (No. 1), 9 (1973) by Tang et al; Levine et al, Appl. Phys. Lett, 26, 375(1975); Appl. Phys. Lett, 37(7), 607(1980) by Feng et al; and U.S. Pat. Nos. 3,384,433; 3,407,309; 3,688,124; 3,842,289; 3,935,472; and 4,054,362.
The thin film waveguides with a periodically modulated nonlinear optical coefficient as described in the literature of interest are either inorganic optical substrates with disadvantages as previously described; or they are organic materials which are in the liquid phase, such as a liquid crystal medium or a thin film of nitrobenzene which require a continuously applied external DC electric field.
There is continuing interest in the development of a short wavelength laser module suitable for a transportable optical disk data recording system.
Accordingly, it is an object of this invention to provide a short wavelength laser source by the frequency doubling of an input laser beam.
It is another object of this invention to provide a short wavelength laser source by efficient frequency conversion of an input long wavelength laser beam in an organic nonlinear optical wavelength.
It is a further object of this invention to provide an all-optical polymeric nonlinear optical waveguide with a spatial periodic structure for modulation of second order susceptibility, and quasi-phase matching for frequency doubling of an input 700-1300 nm laser beam.
Other objects and advantages of the present invention shall become apparent from the accompanying description and drawings.